either way, when I just start to roll from a stop I get a vibration/thump/grinding like sensation about every 2-3 feet no vibration at speed and its intermittant, wont do it every time and I can do nothing to make it happen. I suspect the ujoints since 2 years ago I got a junkyard rearend and had it fixed as to where it should not come apart. Ujoints have to my knowledge ever been greased or replaced.

If it is the rear end I was going to go to 321's instead of the 355's I drive daily on the interstate and do not pull more than 1k pounds which is mt little boat. according to the calculators it should save me 350 rpm's at 60mph. I am guessing that my mpg will go up. Any thoughts?

 

Well, you don't have a transfer case... so, I won't suggest that...

Check your u-joints. They may be dry/binding.

Don't know about your truck, but, at 60MPH, I am barely hitting 1600 on the tach. Drop that down much further, in fourth gear.... with TCC locked..... and it is gonna be a dog. You might actually get WORSE gas mileage. A rather expensive experiment....

 

good point I was wondering about that, 321's were an option when my truck was new, any chance there might be a diffrent transmission or sensor to go with the 321's?

 

The way the speedo gets its signal on your truck, changing gears won't have any effect. Changing tire size will though.

The loss of fuel economy would be due to the engine NOT being in, or, at least, close to... its power band while cruising. I have been unable to find a good dyno chart of a stock motor that actually starts below 2500 RPM.... which renders them pretty much useless for this purpose..... Although... upon investigation, it seems that max torque is achieved right around 3200 RPM...... which would imply that at the 1200 or so you would be turning with 3.21 gears @ 60MPH..... you would be seriously lugging the engine. Not good for gas mileage....

 

oh, I understand now. Yeah, does not sound very good......I guess I will stick to my 355's

 

Plus with 3.55's you can just go grab any lower miles 9 1/4 rear end out of a JY for the price of the install kit to rebuild...

 

well if you decide on the 3.21's..... here you go!

 

thanks for the link

 

Just thinking here, how would a 1500 long bed run with such small gear ratio? What size tires do you run 245 or less?

 

Well I pulled the cover yesterday, everything looked great in the rear end, Will check ujoints this weekend.

I run the stock tire size. As for the 3.21 gears, I noticed at 45ish I can hit od my rpm is 1300-ish which is what it would be with the 3.21 gears if I had them, its a dog at that rpm range with careful throttle control and not hitting passing gear it took forever to move it from 45 to 60. After doing further research, 3.21 gears were an option on these trucks but only for v6 units.

Bottom line, 3.21 likely will not work for a 5.9 any savings from rpm loss will be destroyed by having to mash the gas to get it to go. IMO